Treating aluminum workpieces

ABSTRACT

Surface defects on rolled alulminium alloy sheet, particularly that intended for use as lithographic plate substrate, are caused by copper-containing particles. The invention provides a method of improving the sheet by removing the particles by anodising in an aggressive electrolyte, particularly a.c. anodising at a current density of at least 2 kAm −2 .

Rolled aluminium alloy sheet is extensively used as lithographic platesubstrate, for which purpose it is finally processed by tensionlevelling and cleaning. On being electrograined in nitric acid, surfacedefects may arise which show up as ungrained mirror-like areas,typically 1-2.0 mm in diameter, in a matt grained surface and which leadto large rejection rates. One such area per 20 m² of rolled sheet canlead to rejection of the strip. This is an increasing problem becauseinspection is becoming more rigorous and graining is lighter.

This invention results from the inventors' discovery that these surfacedefects result from the presence of particles more noble than Al on thesurface of the Al workpiece. Such particles most usually contain copperor consist of copper. The actual quantity of copper-containing metaldeposited overall is very small and is extremely difficult to detect inthe rolling production stages. Other contaminant metal particles arepossible. This invention addresses the problem of surface defects in Alsheet by removal of metal particles contaminating the surface thereof.Removal of such particles is preferably effected at a late stage inproduction, after any likely sources of contamination have been passed.Of course, rolled Al sheet is cleaned, particularly for lithographic usebut also for all other purposes; but it has been found that cleaningtechniques in current use may not be effective to remove surface metalparticles.

The invention provides a method of treating an Al workpiece to improve asurface thereof, which method comprises removing noble particles, e.g.Cu-containing particles present on the surface. Preferably removal iseffected by subjecting the Al workpiece to electrolytic treatment, e.g.anodising the Al workpiece in an electrolyte capable of dissolving themetal particles. Preferably the Al workpiece is anodised at a currentdensity of at least 2 kAm⁻².

The same particles may initiate corrosion in rolled sheet intended to bepainted for architectural or automobile use; and in rolled sheet towhich anodic oxide films or organic coatings are intended to be applied.

The workpiece is preferably rolled sheet or strip. The term Al is hereinused to denote pure aluminium metal and alloys containing a majorproportion of aluminium. While the invention is believed applicable toAl alloys generally, it is of particular importance in relation to 1000and 3000 series alloys (of the Aluminum Association Inc. Register)intended for use as lithographic substrates, and also 5000 and 6000series alloys intended for architectural or vehicle or other use.

The electrolyte, which needs to be capable of dissolving the metalparticles, may be acidic or alkaline. Caustic soda and caustic potashare possible. Sulphuric acid is a possible electrolyte, optionallycontaining HF or other additives as in the cleaning fluid marketed byHenkel under the trademark Ridolene 124/120E. Preferred electrolytes arebased on phosphorus oxyacids. This family of acids includesorthophosphoric acid H₃PO₄; metaphosphoric acid and pyrophosphoric acidbased on P₂O₅; and also phosphorous or phosphonic acid H₃PO₃;hypophosphorous or phosphinic acid H₃PO₂; and perhaps others. Aselectrolytes with dissolving power for Cu (and for aluminium oxide) theyall have generally similar properties.

Contamination of the sheet can occur at any stage in the rolling orhandling process but is most likely to occur during hot rolling. Theprocess according to the invention is preferably carried out after hotrolling has been completed. Lithographic sheet is normally cleaned aftercold rolling to final gauge. The present treatment can be applied atthat stage. However, there are practical advantages to removingcontamination by cleaning at an earlier stage either on completion ofhot rolling or at an intermediate stage in the cold rolling, for exampleafter an intermediate anneal. Cleaning at this earlier stage has thefollowing advantages:

1. The contaminating particles are less likely to be firmly rolled intothe surface and therefore are more easily removed.

2. A portion of each contaminating particle becomes smeared over thesurface as cold rolling proceeds and this smear increases the size ofeach resulting area to be removed.

3. As the sheet is cold rolled to progressively thinner gauge, thesurface area to be cleaned increases resulting in increased cost ofcleaning.

Cleaning at an earlier stage in the process does increase the risk ofcontamination arising later in the process remaining in place. However,this risk may be outweighed by the advantages listed above. Of course,the cleaning process can be repeated at later stages in the process andmay in any case be followed by a conventional lighter cleaningoperation.

The method involves anodising the Al workpiece, using either directcurrent or more preferably alternating current. When a.c. is used, it issupposed that electrolysis of the metal particles occurs when the Alsurface is anodic. In addition, when the Al surface is made cathodic,copious quantities of hydrogen gas are formed all over the surface andblow loose debris off. The anodic action can also help to loosenparticles of detritus by undercutting the surrounding Al substrate.

The a.c. wave form may be sinusoidal or not as desired. The a.c. currentmay be biased in either the cathodic or anodic direction. The a.c.frequency is at least several cycles per second and is preferably thecommercial frequency.

Alternatively d.c. anodising may be used. While this is effective toloosen or dissolve metal particles, there is some risk that particlesmay be re-deposited. This risk can be avoided by causing the electrolyteto flow across the surface of the workpiece or by rapidly removing theworkpiece from the electrolyte. Alternatively, d.c. anodisation for aperiod sufficient to loosen metal particles on the surface of the Alworkpiece, can be followed by making the workpiece cathodic for a shortperiod sufficient to generate a burst of hydrogen gas and blow theloosened particles away from the surface. Preferably the workpiece isremoved from the bath under anodic conditions.

The concentration of phosphoric acid, or other electrolyte, ispreferably from 5-30%, particularly 10-25% and more particularly 15-25%e.g. 20%. At low concentrations, the power of the acid to dissolve orloosen metal particles may not be sufficient. At high concentrations,the electrolyte may be so viscous that carry-over of electrolyte becomesa problem, particularly in continuous operations involving immersion forshort periods.

The electrolyte temperature is preferably maintained at 50-100° C. Below50° C., the dissolving power of the electrolyte may be too low. Althoughthere is no theoretical upper limit of temperature, it is in practiceinconvenient to heat phosphoric acid or other electrolytes totemperatures above 100° C. The preferred temperature for a phosphoricacid electrolyte is 80-100° C. e.g. 90° C. At temperatures of 70° C. andabove, anodising can be performed under conditions to remove analuminium oxide film from the surface of the workpiece, thus effectivelycleaning the workpiece, and the treatment to remove metal particlesaccording to this invention can thus be carried out in conjunction withcleaning. At temperatures in the range 50-80° C. (preferably 50-70° C.for Mg containing alloys) anodising can be performed under conditions tocreate or maintain an anodic aluminium oxide film, and this may increasethe surface resistance of the Al workpiece and favour a current paththrough the metal particles. Thus operating under conditions to create,rather than remove, anodic aluminium oxide helps to remove the metalparticles by electrolysis. The anodic film may be completely orpartially dissolved if the strip is left in the electrolyte away fromthe influence of the electrodes.

A relatively high current density of at least about 2 kAm⁻² is preferredto remove metal particles. This is higher than the current densitiesordinarily used when anodising or cleaning Al surfaces.

Treatment time can be very short e.g. as low as 0.1 s. It is envisagedthat treatment will be performed by passing rolled strip continuouslythrough a treatment bath which may, depending on other production lineparameters, need to be done at high speed. The treatment time is thusthe time spent in the electrolyte. Treatment times are preferably in therange of 0.5-30 s. The period of time during which the workpiece is inthe vicinity of the electrodes and undergoes electrolytic treatment maybe less than the total treatment time, and is preferably at least 0.25s, in particular in the range 0.25-15 s or 0.25-5 s or 0.25-3 s e.g.around 0.5 s The total charge input is expected to be in the range of0.2-50 or 0.2-30 kCm⁻² e.g. around 1 kCm⁻².

Preferred conditions for operating the process according to theinvention are summarised as follows:

A.C. electrolytic treatment for at least 0.25 seconds under theelectrode preferably 0.25-3 seconds e.g. around 0.5 s.

Phosphoric acid electrolyte at 80-100° C. e.g. 90° C.

Acid concentration in the electrolyte 15-25% e.g. 20%.

Current density at least 2 kAm⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is directed to the accompanying drawings, in which each ofFIGS. 1 to 11 is a micrograph, or a set of micrographs, of an Al alloysurface contaminated with Cu-containing particles. In these micrographs,the Al metal surface appears as a grey streaked background.Cu-containing particles appear white. SiC particles, which are anartefact of the experimental technique used, appear dark.

The following examples illustrate the invention.

EXAMPLE 1

Electrolytic vs Acid Etch

To demonstrate the efficiency of cleaning coil contaminated with coppercontaining particles, two samples of final gauge 0.3 mm coil wereimpregnated with fine copper and 70/30 brass particles by lightlyrolling them into the surface. The particles were produced by abradingcopper and brass with silicon carbide paper. Sufficient samples of eachparticulate could then be collected, although some transfer of thesilicon carbide abrasive also occurred.

Anodising was effected using a 20% phosphoric acid electrolyte at 80° C.for three seconds at 3 volts. The comparative experiment used theproprietary etch Ridolene 124/120E, which is a 0.5% sulphuric acid, withdispersants and 300 ppm HF and one of the fastest proprietary etches.Samples were immersed for a period of 60 seconds at 60°0 C. A total of 6samples were produced, these are:

FIG. 1: Brass particles rolled into 1050A alloy.

FIG. 2: Brass particles rolled into 1050A alloy Ridolene cleaned.

FIG. 3: Brass particles rolled into 1050A alloy phosphoric acidanodised.

FIG. 4: Copper particles rolled into 1050A alloy.

FIG. 5: Copper particles rolled into 1050A alloy Ridolene cleaned.

FIG. 6: Copper particles rolled into 1050A alloy phosphoric acidanodised.

The samples were examined with a scanning electron microscope using aback-scattered detector. FIG. 1 shows the frequency of the number ofbrass particles in the as rolled condition only. The darker-than-matrixparticles of silicon carbide can also be seen. After cleaning inRidolene for 60 seconds, (FIG. 2) all of the brass particles stillremain. However the 3 second phosphoric acid anodisation has removed themajority of particles (including many of the coarser silicon carbideparticles) with only one brass particle remaining as shown in FIG. 3. Asimilar story exists for the rolled-in copper particles. FIG. 4 givesdetail of the rolled-in copper particles before cleaning. Again theRidolene clean shows little effect on the removal of the copperparticles (FIG. 5) but in contrast the three second phosphoric acidanodisation has removed nearly all of the particles as is shown in FIG.6.

This simulation demonstrates the efficiency of the phosphoric acidanodisation in removing the copper containing particles versus theRidolene clean.

EXAMPLE 2

Cleaning vs Anodising

It was thought that by anodising (at 60° C.) rather than cleaning (at80° C.) the surrounding aluminium surface would be rendered slightlymore passive and allow the cleaning action to concentrate on the copperparticles.

Samples of 1050A final gauge 0.3 mm coil were impregnated with finecopper particles as before. They were then cleaned or anodised underconditions that simulate commercial conditions, e.g. 20% phosphoric acidelectrolyte at 80° C. and 60° C. respectively for 0.5 seconds. Threedifferent a.c. voltage levels were employed namely 3, 7 and 15 volts(FIGS. 7, 8 and 9 respectively). In order to determine the passivatingeffect of the film generated by the two processes samples were immersedin a 3% NaOH solution at 60° C. and the time was measured until gassingoccurred. In all cases the times were acceptably small (1.5-3.7 s)indicating that passivation is not a problem.

Each surface to be treated was initially characterised using the SEM,and after treatment the same area was examined. The SEM examination wasdone using the back-scattered detector so that the higher atomic numbercontrast of any remaining copper found on the surface after treatmentcould be observed.

The SEM photographs are shown in FIGS. 7 to 9. The top set ofphotographs are before treatment and the corresponding bottom set areafter cleaning (80° C.) or anodising (60° C.). It was found that at the15 volt treatment (FIG. 9) the particles were most effectively removedin 0.5 sec. There was no observable difference between cleaning andanodising, however there was a greater whitening effect generated by thecleaning treatment.

The applied current for the 15 volt 60° C. condition was 2300 Amps/m²and for the 80° C. condition the applied current was 3700 Amps/m².

EXAMPLE 3

DC Anodising

Anodising 1050A alloy with d.c. removes the particles but tends toresult in copper particles being redeposited. A current density of 3000Amps/m² was most favourable, FIG. 10 where the top micrograph is of thesample as received and the other two show, at different magnifications,the surface after d.c. anodic cleaning. However the copper which hasgone into solution has, at least in part, been redeposited on thesurface. Cathodic d.c. was not effective at removing the copperparticles even at 3000 Amps/m², FIG. 11 where the top micrograph is ofthe sample as received and the bottom one shows the surface after d.c.cathodic cleaning. This tends to prove that removal is primarily byelectrolysis.

EXAMPLE 4

A.C. Cleaning On A Continuous Line

A strip of AA1050A material was passed through two cleaning cellscontaining 18% phosphoric acid at 90° C., which applied power in theliquid contact mode. The line speed was 40 m/min. the strip width was1.37 m and the gauge 2.2 mm, that is, the coil was treated afterinterannealing, but before further cold rolling to a final gauge of0.275 mm. the current and charge densities used were 2.3 kA/m² and 5.5kCoulombs/m² respectively and the voltage applied was 24 volts. Thenumber of defects detected after graining in nitric acid under normalcommercial conditions was ten times less than in identical materialrolled and cleaned under standard commercial conditions. Furtheroptimisation of the cleaning step is expected to reduce the number ofdefects still further.

What is claimed is:
 1. A method of treating an Al workpiece to improve asurface thereof, which method comprises removing particles more noblethan aluminium present in the surface, wherein the particles are removedby subjecting the Al workpiece to electrolytic treatment in a phosphorusoxyacid electrolyte.
 2. A method as claimed in claim 1, wherein theparticles are contaminant metal particles.
 3. A method as claimed inclaim 2, wherein the particles are copper-containing particles.
 4. Amethod as claimed in claim 1, wherein the particles arecopper-containing particles.
 5. A method as claimed in claim 1, whereinremoval is effected by anodising the Al workpiece in an electrolytecapable of dissolving the metal particles.
 6. A method as claimed inclaim 4, wherein the Al workpiece is anodised at a current density of atleast 2 kAm⁻².
 7. A method as claimed in claim 5, wherein a.c. anodisingis used.
 8. A method as claimed in claim 4, wherein d.c. anodising isused with the electrolyte being caused to flow across the surface of theworkpiece.
 9. A method as claimed in claim 1, wherein the Al workpieceis of a 1000 or 3000 series alloy of the Aluminum Association Inc.Register.
 10. A method as claimed in claim 1, wherein the Al workpieceis rolled metal sheet for use as a lithographic plate support.
 11. Amethod as claimed in claim 1, wherein the Al workpiece has been rolledprior to the step of removing particles present in the surface, and isagain rolled after the said step.